The present invention generally relates to position detection apparatus, and more particularly to apparatus for determining the position of a member that is movable along a defined path of finite length.
It is often desirably to identify the position of a device that is controlled by an actuator or the like. For example, in the controls industry, devices such as valves having valve stems or valve shafts that are movable by an actuator are used to control the flow of liquids and gases associated with industrial processes of various types. In these applications, it is often desirable to identify, at any given time, the precise position of the movable valve stem or valve shaft. This information allows improved understanding and control of the process.
A number of prior solutions have been proposed. Optical coding schemes make use of a coded element with opaque and transparent sections to provide digital data inputs to an array of sensors positioned to measure the light passing through the sections. While optical coding devices do not require a mechanical linkage, the optical approach only works well in very clean environments and is therefore not applied in many industrial environments.
Linear variable differential transformers (lvdt) can provide very accurate position information, but typically require a mechanical linkage and also generally use relatively high power. Potentiometers or other rotary transducers often require a mechanical linkage and also have the disadvantage of a sliding electrical contact that can cause long-term reliability issues. Hall effect transducers, as they are currently used, generally require a mechanical linkage.
An improved approach for determining the position of a movable member is disclosed in U.S. Pat. No. 4,698,996 to Kreft et al. Kreft et al. suggest providing a bar magnet on the movable member, which then moves parallel to a plurality of spaced sensors. During a calibration procedure, the bar magnet is moved step-by-step in a direction parallel to the line of sensors in precisely defined length units. When an output voltage of a particular sensor is zero, and neighboring sensors on either side thereof have respective positive and negative values, a length value is assigned to the particular sensor and stored.
For unknown positions of the magnet, the voltage values of neighboring sensors that are influenced by the magnet are measured and the relationships thereof are determined. Adjacent sensors that have voltage values that are of different polarity are selected. For voltage relationships that correspond exactly to a calibrated voltage relationship, the corresponding calibrated positional value is assigned to the unknown position. For voltage relationships lying between the calibration values, suitable interpolation methods are used to define the position of the magnet.
A limitation of Kreft et al. and others is that the spacing between neighboring sensors must typically be relatively small. This is because the range over which the sensors can provide a linear output signal is limited. Accordingly, even when the travel distance of the magnet is limited, a significant number of sensors may be required. This can significantly increase the cost of the position-determining device.
Therefore, a need exists for a position determining apparatus that does not require such a small sensor spacing, while still reliably and accurately determining the position of the magnet.
The present invention solves these and other needs by providing a sensor that has an increased linear range. This can reduce the number of sensors that are required, and can improve the accuracy of the device. This is preferably accomplished by providing a distributed bridge sensor that has one or more magneto-resistive elements positioned at a first location along the defined path of the magnet, and one or more other magneto-resistive elements positioned at a second location along the defined path. The magneto-resistive elements at the first location thus experience a different magnetic field component than those at the second location. It has been found that the output of such a distributed bridge sensor has an increased linear range relative to the non-distributed bridge sensors of the prior art.
The present invention also contemplates providing a first non-distributed bridge sensor at the first location and a second non-distributed bridge sensor at the second location. A compensation signal may then be generated by differencing the output of the first and second non-distributed bridge sensors. The compensation signal is relatively constant over much of the linear range of the distributed bridge sensor, and can be used to compensate the sensitivity of the distributed bridge sensor for variations in temperature, voltage, etc. The first and second non-distributed bridge sensors also can detect over-travel of the magnet relative to the distributed bridge sensor. When the magnet travels outside of the linear range of the distributed bridge sensor, a controller can set the output signal to a predetermined value to indicate the over-travel condition, and/or activate a neighboring pair of sensors to continue monitoring the position of the magnet.
The distributed bridge sensor can be implemented in a variety of ways. In a first illustrative embodiment, the distributed bridge sensor is formed from two distributed half-bridge sensors. First and second magneto-resistive elements are positioned at a first location along the defined path of the magnet, and third and fourth magneto-resistive elements are positioned at a second location.
The first and fourth magneto-resistive elements are coupled together in a half-bridge configuration to provide a first output signal. Likewise, the third and second magneto-resistive elements are coupled together in a half-bridge configuration to provide a second output signal. A differencing circuit, such as a differential amplifier, may then be used to provide a magnet position signal. The magnet position signal is derived from the difference between the first output signal and the second output signal.
Preferably, the first and second magneto-resistive elements are provided in a first integrated circuit package, and the third and fourth magneto-resistive elements are provided in a second integrated circuit package, although this is not required. In one embodiment, the first and second magneto-resistive elements are collectively rotated relative to the second and third magneto-resistive elements. More specifically, the first and second magneto-resistive elements are collectively rotated toward a center magnetic position between the first and second magneto-resistive elements and the third and fourth magneto-resistive elements. Likewise, the third and fourth magneto-resistive elements are collectively rotated toward the center magnet position.
Alternatively, the distributed bridge sensor may be formed from two distributed full bridge sensors. In this embodiment, first and second magneto-resistive elements are positioned at the first location, and third and fourth magneto-resistive elements are positioned at a second location. In addition, however, fifth and sixth magneto-resistive elements are positioned at the first location, and seventh and eighth magneto-resistive elements are positioned at the second location.
The first magneto-resistive element and the second magneto-resistive element are coupled together in a half-bridge configuration, and provide a first output. The third and fourth magneto-resistive elements are also coupled together in a half-bridge configuration, and provide a second output. The first output and the second output are coupled together to provide a first full bridge output.
Likewise, the fifth and sixth magneto-resistive elements are coupled together in a half-bridge configuration, and provide a third output. The seventh and eighth magneto-resistive elements are also coupled together in a half-bridge configuration, and provide a fourth output. The third output and the fourth output are coupled together to provide a second full bridge output. A differencing circuit, such as a differential amplifier, is then used to provide a magnet position signal that is derived from the difference between the first full bridge output signal and the second full bridge output signal.
Like above, a first over-travel sensor may be provided at the first location for detecting when the magnet passes a first over-travel location along the defined path. Likewise, a second over-travel sensor may be provided at the second location for detecting when the magnet passes a second over-travel location along the defined path. The first and second over-travel location preferably define the linear range of the distributed bridge sensor. When the magnet travels outside of the linear range of the distributed bridge sensor, a controller set the output signal to a predetermined value to indicate the over-travel condition, and/or activates a neighboring sensor pair to continue monitoring the position of the magnet.
The first and second over-travel sensors are preferably non-distributed bridge sensors. A compensation signal may be generated by differencing the output of the first and second over-travel sensors. The compensation signal is relatively constant over much of the linear range of the distributed bridge sensor, and can be used to compensate the sensitivity of the distributed bridge sensor for variations in temperature, voltage, etc.
Methods for determining a position of a magnet movable along a defined path are also contemplated. An illustrative method includes the steps of: sensing the position of the magnet using a distributed bridge sensor; and providing an output signal that is related to the position of the magnet. The method may further include the steps of: sensing the position of the magnet using a first non-distributed bridge sensor, wherein the first non-distributed bridge sensor is located at a first location along the defined path; and sensing the position of the magnet using a second non-distributed bridge sensor, the second non-distributed bridge sensor located at a second location along the defined path.
In providing an output signal, the method may further include the steps of passing a first predetermined voltage to the output terminal when the position of the magnet is left of a predetermined left over-travel location; passing a second predetermined voltage to the output terminal when the position of the magnet is right of a predetermined right over-travel location; and passing the magnet position signal to the output terminal when the position of the magnet is between the predetermined left over-travel location and the predetermined right over-travel location. The first non-distributed bridge sensor may detect when the position of the magnet is left of the predetermined left over-travel location, and the second non-distributed bridge sensor may detect when the magnet is right of the predetermined right over-travel location.
Finally, to compensate for changes in temperature, voltage, etc., the method may include the steps of determining the difference between the position of the magnet sensed by the first non-distributed bridge sensor and the position of the magnet sensed by the second non-distributed bridge sensor, thereby resulting in a measured difference value; comparing the measured difference value to a predetermined compensation value; and changing the supply voltage of the distributed bridge sensor until the measured difference value substantially equals the predetermined compensation value.